The inventive subject matter relates to motor control apparatus and methods of operation thereof, more particularly, to motor starters and methods of operating the same.
Motor starters are commonly used with industrial electric motors. Typical solid-state motor starters control starting characteristics to meet application requirements, including acceleration and deceleration time, starting current and motor torque. Soft starters are commonly used to limit inrush current when the motor is first coupled to a power source. Large inrush currents may cause voltage dips that may negatively affect other loads coupled to the same source.
Soft motor starters may control voltage and/or current provided to the motor. For example, some soft starters may use silicon-controlled rectifiers (SCRs) that are connected in series between the power source and the load. During a startup process, the SCRs may be selectively gated “on” to gradually increase the speed of the motor. When the motor has reached a desired speed, a bypass contactor may be closed to bypass the SCRs and reduce losses. Operations of soft motor starters are described in “Solid-state soft start motor controller and starter,” Application Paper AP 03902001E (Eaton Corp. 2011).
When a motor is started with a reduced voltage soft starter (RVSS), the motor current may be limited to prevent a sag or brownout in the line voltage provided to the starter. Commonly, a current limit of the starter may be adjusted to achieve a value that is suitable for the site conditions, e.g., a value that provides a desirable starting performance without introducing an undesirable degree of input voltage dip. However, line conditions may be subject to change due to stiffness of the utility source and system loading conditions. This often results in setting the current limit at a level that corresponds to a worst case line condition, which can result in suboptimal starting performance.